I love Firefox "reader mode" and wish there were some way to turn it on by default, because it always provides a better experience than whatever the page designer intended.
https://github.com/mozilla/readability/blob/master/Readabili...
which is a descendant of the original Readability.js published by Arc90, before they decided to turn it into a proprietary Instapaper clone.
It extracts what looks like article text from the page markup. DOM elements included for a printer-friendly view could possibly be helping, but it doesn't target that directly.
I suspect it's looking at the page before it's fully rendered and determining that it doesn't meet some criterion for reader mode.